Among Linux kernel developers, the -mm tree refers to a set of patches to the kernel source code maintained by Andrew Morton. The -mm kernel git tree now fills the role of Linux kernel development builds, formerly identified by odd minor version numbers (see this section on Linux kernel version numbering). Currently new and experimental code finds its way into a 2.6.x-mm kernel build. Historically the -mm tree focused on new developments for the memory management part of the kernel (mm). Occasionally, the -mm tree was overloaded with new patches so testing it became overly difficult. On Sep 17 2007 Andrew Morton sent a mail saying that "this just isn't working any more".
